Ryuji Morishima

N-body simulations of oligarchic growth of Mars: Implications for Hf–W chronology

Dauphas and Pourmand [2011. Hf–W–Th evidence for rapid growth of Mars and its status as a planetary embryo. Nature 473, 489–492] estimated the accretion timescale of Mars to be 1.8−1.0+0.9Myr from the W isotopes of Martian meteorites. This timescale was derived assuming perfect metal–silicate equilibration between the impactor and the targets mantle. However, in the case of a small impactor most likely only a fraction of the targets mantle is involved in the equilibration, while only a small part of the impactors core equilibrates in the case of a giant impact. We examined the effects of imperfect equilibration using results of high-resolution N-body simulations for the oligarchic growth stage. These effects were found to be small as long as a planetary embryo has a deep liquid magma ocean during its accretion. The effect due to partial involvement of the targets mantle in equilibration is small due to the low metal–silicate partition coefficient for W suggested from the low Hf/W ratio of the Martian mantle. The effect due to partial involvement of the impactors core is also small because a large fraction of the embryo mass is delivered from small planetesimals, which are likely to fully equilibrate in the deep magma ocean on the embryo. The accretion timescale of Mars estimated by the Hf–W chronology is shorter than that expected for the minimum mass solar nebula model as long as more than 10% of each impactors core re-equilibrates with the Martian mantle and the final stages of accretion are prolonged. This probably indicates that accretion of Mars rapidly proceeded due to solid and gas surface densities significantly larger than those for the minimum mass solar nebula or due to accretion of small fragments or pebbles.

Gap opening beyond dead zones by photoevaporation

We propose a new hypothesis for the origin of protoplanetary discs with large inner holes (or gaps), so-called transition discs. Our gas disc model takes into account layered accretion, in which poorly-ionized low-viscosity dead zones are sandwiched by high-viscosity surface layers, and photoevaporative winds induced by X-rays from the central stars. We find that a gap opens at a radius outside a dead zone, if the mass loss rate due to photoevaporative winds exceeds the mass accretion rate in the dead zone region. Since the dead zone survives even after the gap opens, mass accretion onto the central star continues for a long time. This model can reproduce large gap sizes and high mass accretion rates seen in observed transition discs.

Two types of co-accretion scenarios for the origin of the Moon

Based on orbital calculations of Keplerian planetesimals incident on a planet with various initial orbital elements, we develop a numerical model which describes the accretional and dynamical evolution of planet-satellite systems in a swarm of planetesimals on heliocentric orbits with given spatial and velocity distributions. In the plane of orbital radius of the satellite vs. satellite/planet mass ratio, a satellite with some initial value moves quickly toward the balanced orbital radius, where accretion drag compensates with tidal repulsion, and then grows toward the equilibrium mass ratio. Using the model, we propose two types of co-accretion scenarios for the origin of the Moon, both of which satisfy the most fundamental dynamical constraints: the large angular momentum of the Earth-Moon system and the large Moon/Earth mass ratio. In the first scenario the Moon starts from a small embryo and grows in a swarm of planetesimals with low velocity dispersion and nonuniform spatial distribution, so that large spin angular momentum is supplied to the planet. Such a situation would be realized when the Earth grows up rapidly before dissipation of the solar nebula. Second one considers co-accretion after a giant impact during Earth accretion, which produces enough angular momentum as large as that of the present Earth-Moon system as well as a lunar-sized satellite. In this case, solar nebula would have already dissipated and random velocities of incident planetesimals are rather high, so that the Earth grows slowly. We find that the total angular momentum decreases by 5–25% during this co-accretion stage.

Onset of oligarchic growth and implication for accretion histories of dwarf planets

Abstract We investigate planetary accretion that starts from equal-mass planetesimals using an analytic theory and numerical simulations. We particularly focus on how the planetary mass M oli at the onset of oligarchic growth depends on the initial mass m 0 of a planetesimal. Oligarchic growth commences when the velocity dispersion relative to the Hill velocity of the protoplanet takes its minimum. We find that if m 0 is small enough, this normalized velocity dispersion becomes as low as unity during the intermediate stage between the runaway and oligarchic growth stages. In this case, M oli is independent of m 0 . If m 0 is large, on the other hand, oligarchic growth commences directly after runaway growth, and M oli ∝ m 0 3 / 7 . The planetary mass M oli for the solid surface density of the Minimum Mass Solar Nebula is close to the masses of the dwarf planets in a reasonable range of m 0 . This indicates that they are likely to be the largest remnant planetesimals that failed to become planets. The power-law exponent q of the differential mass distribution of remnant planetesimals is typically − 2.0 and − 2.7 to − 2.5 for small and large m 0 . The slope, q ≃ − 2.7 , and the bump at 10 21 g (or 50 km in radius) for the mass distribution of hot Kuiper belt objects are reproduced if m 0 is the bump mass. On the other hand, small initial planetesimals with m 0 ∼ 10 13 g or less are favored to explain the slope of large asteroids, q ≃ − 2.0 , while the bump at 10 21 g can be reproduced by introducing a small number of asteroid seeds each with mass of 10 19 g.

Incomplete cooling down of Saturn’s A ring at solar equinox: Implication for seasonal thermal inertia and internal structure of ring particles

Abstract At the solar equinox in August 2009, the Composite Infrared Spectrometer (CIRS) onboard Cassini showed the lowest Saturn’s ring temperatures ever observed. Detailed radiative transfer models show that the observed equinox temperatures of Saturn’s A ring are much higher than model predictions as long as only the flux from Saturn is taken into account. In addition, the post-equinox temperatures are lower than the pre-equinox temperatures at the same absolute solar elevation angle. These facts indicate that the A ring was not completely cooled down at the equinox and that it is possible to give constraints on the size and seasonal thermal inertia of ring particles using seasonal temperature variations around the equinox. We develop a simple seasonal model for ring temperatures and first assume that the internal density and the thermal inertia of a ring particle are uniform with depth. The particle size is estimated to be 1–2 m. The seasonal thermal inertia is found to be 30–50 J m−2 K−1 s−1/2 in the middle A ring whereas it is ∼10 J m−2 K−1 s−1/2 or as low as the diurnal thermal inertia in the inner and outermost regions of the A ring. An additional internal structure model, in which a particle has a high density core surrounded by a fluffy regolith mantle, shows that the core radius relative to the particle radius is about 0.9 for the middle A ring and is much less for the inner and outer regions of the A ring. This means that the radial variation of the internal density of ring particles exists across the A ring. Some mechanisms may be confining dense particles in the middle A ring against viscous diffusion. Alternatively, the (middle) A ring might have recently formed (<108 yr) by destruction of an icy satellite, so that dense particles have not yet diffused over the A ring and regolith mantles of particles have not grown thick. Our model results also indicate that the composition of the core is predominantly water ice, not rock.

A particle-based hybrid code for planet formation

Abstract We introduce a new particle-based hybrid code for planetary accretion. The code uses an N -body routine for interactions with planetary embryos while it can handle a large number of planetesimals using a super-particle approximation, in which a large number of small planetesimals are represented by a small number of tracers. Tracer–tracer interactions are handled by a statistical routine which uses the phase-averaged stirring and collision rates. We compare hybrid simulations with analytic predictions and pure N -body simulations for various problems in detail and find good agreements for all cases. The computational load on the portion of the statistical routine is comparable to or less than that for the N -body routine. The present code includes an option of hit-and-run bouncing but not fragmentation, which remains for future work.

Seasonal variation of the radial brightness contrast of Saturn’s rings viewed in mid-infrared by Subaru/COMICS

Aims. This paper investigates the mid-infrared (MIR) characteristics of Saturn’s rings. Methods. We collected and analyzed MIR high spatial resolution images of Saturn’s rings obtained in January 2008 and April 2005 with the COoled Mid-Infrared Camera and Spectrometer (COMICS) mounted on the Subaru Telescope, and investigated the spatial variation in the surface brightness of the rings in multiple bands in the MIR. We also composed the spectral energy distributions (SEDs) of the C, B, and A rings and the Cassini Division, and estimated the temperatures of the rings from the SEDs assuming the optical depths. Results. We found that the C ring and the Cassini Division were warmer than the B and A rings in 2008, which could be accounted for by their lower albedos, lower optical depths, and smaller self-shadowing effect. We also fonud that the C ring and the Cassini Division were considerably brighter than the B and A rings in the MIR in 2008 and the radial contrast of the ring brightness is the inverse of that in 2005, which is interpreted as a result of a seasonal effect with changing elevations of the Sun and observer above the ring plane.